The present invention relates to a multi-layer tubular film and a method of making the film by extrusion. More particularly, the present invention relates to a multiple layer extrudate having a core layer which provides a substantially uniform, relatively low oxygen transmission rate therethrough around the entire periphery of the film, including the weld line produced during extrusion of the film.
Film structures including a polyvinylidene chloride layer have in the past been difficult to extrude into tubular form. Extrusion of polyvinylidene chloride, such as Saran (manufactured by The Dow Chemical Company), presents special problems due to the thermal degradation which can occur in the extrusion die. It has been found, however, that by using a multi-component feedblock process in which a polyvinylidene chloride is in the center of the feedblock and therefore moves quickly through the die, extrusion of this material is possible. The polyvinylidene chloride core is, then, encapsulated by a different material, such as a polyethylene. When the feedblock is extruded, the resulting extrudate includes a middle layer of polyvinylidene chloride.
A problem in forming the encapsulated polyvinylidene chloride into a tubular film by extrusion results from the seam produced during the extrusion process. This seam extends along a weld line on one side of the extrudate. Generally, an extrusion die used to produce the tubular extrudate defines an annular chamber into which the extrusion material is forced. The material divides into two substreams which flow in opposite directions around the annular chamber, meeting on the opposite side of the chamber where they recombine. The streams then exit from the annular chamber through an annular opening and, at the point of recombination, define the weld line.
It will be appreciated that if a feedblock having a core of polyvinylidene chloride and an outer, encapsulating polyethylene layer were to be extruded in this fashion, the polyvinylidene chloride core does not recombine along the weld line. Rather, only the encapsulating material recombines at the weld line. Since polyethylene permits a much higher level of gas transmission therethrough than polyvinylidene chloride, a tubular extrudate having non-uniform gas transmission properties is produced. This is particularly undesirable where the extrudate is to be used for containment of a gas-sensitive material. For example, if the tubular film having a gap in the polyvinylidene chloride layer along its weld line were to be utilized for meat storage, the shelf life of the product would be shortened due to the transmission of oxygen through the film in this region. It will be appreciated that a film having non-uniform gas transmission properties would be unacceptable for this and many other applications.
One solution to this problem, as suggested by U.S. Pat. No. 4,042,661, issued Aug. 16, 1977 to Cook, is to extrude coaxially two multi-layer tubular film structures, with the weld lines of the film structures spaced around the periphery of final tubular film which combines these two structures. While providing a solution to the problem discussed above, the Cook '661 patent requires several polyvinylidene chloride layers. Thus, the structure would be too complicated for some applications and, in any event, would require many more layers than should be necessary.
Accordingly, it is seen that there is a need for an extrudate having a core layer which, in tubular form, provides a generally uniform oxygen transmission level therethrough around its periphery and which is simple in construction.